1. Field of the Invention
This invention relates to a method and apparatus for measuring the cross-sectional or three-dimensional form of an object using an apparatus that has imaging capabilities such as an ITV camera or a CCD camera.
2. Description of the Prior Art
In the steps of manufacturing, processing and inspecting various industrial products, in addition to one or two dimensional data such as width, height and area it is often necessary to measure three dimensional aspects of a product such as its cross-sectional form, in order to determine its roundness, for example. Generally, also, in the case of industrial products it often desirable that the measurement procedures be quick and non-contact.
For such purposes, various form measurement methods have been tried that employ imaging devices such as ITV cameras. Typical examples of these conventional methods will now be described with reference to FIGS. 10 and 11.
FIG. 10 is a drawing showing the general arrangement of a conventional apparatus for measuring the cross-sectional form of an object using a single ITV camera. The apparatus comprises a revolving platform 80 for rotating a measurement object 80 which is mounted on the revolving platform 80; illumination means 81 for illuminating the measurement object 84; an angle of rotation detector section 82 for detecting the angle of rotation of the measurement object 84, that is, the angle of rotation of the revolving platform 84; and an ITV camera 13 located at a position that is a distance B8 from the revolving platform 80. There is also a gear box 83. To obtain an image of the measurement object 84, the object 84 is placed on the revolving platform 80 and the revolving platform 80 is rotated intermittently by predetermined angles of rotation and an image of the object is taken by the ITV camera 13 at each predetermined angle of rotation. For this, the angle of rotation of the revolving platform 80 is measured by the angle of rotation detector section 82.
To obtain stable binary picture images, the measurement object 84 is backlit by means of the illumination means 81 to process the background as "light" and the measurement object as "dark".
The measurement principle of this apparatus is that at each predetermined angle of rotation by which the revolving platform 80 is turned an image of the measurement object 84 is obtained by the ITV camera 13 to thereby provide projection images of the measurement object 84 from numerous directions. These numerous projection images are then used to obtain groups of projection lines that represent functions of the angles of rotation and the distance B8. Because these projection line groups all contact the horizontal cross-sectional contour of the measurement object 84 at the height of the ITV camera 13, that is, the horizontal cross-sectional boundary line of the object, the horizontal cross-sectional form S8 of the measurement object 84 can be determined by obtaining the envelope of the projection line groups.
FIG. 11 is a drawing showing the general arrangement of a conventional apparatus for measuring the cross-sectional form of an object using two ITV cameras.
This measurement apparatus is configured so that images of the upper and lower edges of a measurement object 90 are picked up by two ITV cameras 13 and 14 located on the same line L9 which are separated by a distance B9. To measure cross-sectional contours with this apparatus, the measurement object 90 is rotated intermittently by predetermined angles and at each rotation an image of the upper and lower edges of the object 90 is obtained by the ITV cameras 13 and 14 to thereby obtain diameter values around the perimeter of the measurement object 90.
The center values of the diameter values thus obtained are then superimposed on the coordinate origins and for each of the measured values the coordinates are rotated by an angle that equals each angle of rotation of the measurement object 90. The cross-sectional form S9 of the measurement object 90 is obtained by connecting the points denoting the diameter values obtained by the above operations.
Thus, a number of conventional methods have been developed relating to measuring the form of an object. However, the restrictions and constraints imposed by these methods relating to the object measurement or the measurement environment have limited their range of application.
For example, in the measurement of cross-sectional form using the apparatus shown in FIG. 10, the projection line groups obtained by the imaging are represented as functions of the angle of rotation of the measurement object 84 and revolving platform 80, and the distance B8 between the revolving platform 80 and the camera 13. Therefore, for measuring cross-sectional form with this apparatus it is necessary to know beforehand the angles of rotation of the object and the distance B8.
However, because in ordinary measurement conditions the angle of rotation of the revolving platform 80 and the distance B8 fluctuate owing to play in the revolving platform, vibration in the measurement system and so forth, in many cases these values become indeterminate. That is, form measurement using the apparatus of FIG. 10 is difficult to apply where the measurement system has a fluctuating rotation or translational motion component.
In the measurement of cross-sectional form using the apparatus of FIG. 11, the cross-sectional form is obtained from the diameter values of the measurement object 90, so the influence of translational motion by the measurement object 90 can be reduced. However, in this method also, the angle of rotation of the measurement object 90 has to be known beforehand. Also, the measurement principle cannot be applied to objects with gleichdick forms.
In addition, basically both of the methods illustrated by FIGS. 10 and 11 measure the form using only projection image data of the object, that is, contour tangential data, not picture image data of the object. Because of this, although measurement is possible when the cross-sectional form of the object is convex, the methods are not applicable to concave objects.
Hence, because the conventional methods require restrictions or constraints relating to the object measurement or the measurement environment their range of application has been limited. The need has therefore been to obtain a method and apparatus for measuring form which does not have the conventional restrictions and constraints.